In contrast to work on birds and nonhuman primates there have been few demonstrations of plasticity in nonhuman primate development. We have developed three model systems 1. trill structure in pygmy marmosets where we have found monkeys alter the structure of calls when placed in a new colony (Elowson et al, 1994) or when paired with a new mate (Snowdon et al., in preparation). Recent work on pygmy marmosets in the natural habitat in the Amazon basin finds that monkeys living in different habitats have different trill structures suggesting plasticity in response to environmental variables (S. de la Torre, in progress). 2 Babbling. Pygmy marmosets show a high level of vocal activity from infancy through week 30 long after infants are independent. We have compared this vocal behavior with criteria used to establish babbling in human infants and argue that the parallels between pygmy marmoset "babbling" and human infants are quite strong (Elowson et al., in review). We have done an extensive descriptive analysis of babbling in the first 20 weeks, and find that pygmy marmosets infants produce about 60% of the adult repertoire in babbling, that babbling leads to social responses by adult care-givers, and that the structure of babbling improves toward adult form with increased age (Elowson, Lazaro-Perea, and Snowdon, in preparation). We are currently examining in microscopic detail the types of calls within babbling that elicit caretaker responses and how the structure of babbling changes after social interaction (Emmer and Snowdon, in progress). 3. Food calls We have found that cotton top tamarins have specific calls used in response to food and that in adults these calls are correlated with food preferences. Younger animals use imperfect forms of adult calls and many other calls as well, and they overgeneralize from food to other objects. There was no progressive improvement with increasing age (Roush and Snowdon, 1994). We have completed several other studies showing that call structure and usage changes rapidly when the social status of animals is changed from subordinate non- reproductive to a reproductive animal, and that there is apparent "teaching" of vocalizations by adult animals while sharing food with young (Roush, in preparation). We have also found that food-associated calls exist in pygmy marmosets with similar correlation between call rate and food preferences (Snowdon, Fjelsted and Castro, in preparation). We have also studied whether cotton-top tamarins can distinguish between the scent marks of familiar versus unfamiliar adult females as well as between non-reproductive unfamiliar females and reproductive unfamiliar females. We have tested six pairs of tamarins and find clear evidence of olfactory discrimination of both the reproductive state of unfamiliar females as well as discrimination of familiar from unfamiliar females (Washabaugh and Snowdon, in preparation). Finally a book integrating studies of birds, dolphins, nonhuman primates and humans on the role of social influences on vocal development is currently in press with Cambridge University Press (Snowdon and Hausberger)